1. Field of the Invention
The present invention relates to an anti-skid control system for an electrically operated vehicle to control the braking force applied to road wheels including driven wheels in a braking operation, and more particularly to the anti-skid control system for estimating a coefficient of friction of a road surface immediately after the vehicle is braked, and controlling the braking force in accordance with the estimated coefficient of friction.
2. Description of the Related Arts
In general, the coefficient of friction between the road surface and the road wheels is varied in dependence upon the kinds of the road wheels, the road surface condition and the like. Especially, the coefficient of friction (herein, abbreviated as .mu.) is varied to a large extent in dependence upon the conditions of the road on which the vehicle is running, such as a dry road surface and a wet road surface. Thus, it is very important to detect the coefficient of friction of the road surface (hereinafter, referred to as the road-CF). In this respect, when the vehicle is running, it is impossible to directly detect the road-CF. Therefore, according to an anti-locking control method disclosed in Japanese Patent Laid-open publication No.3-67770 for example, a model vehicle speed is calculated from the maximum of the wheel speeds of the four wheels, and a mean deceleration of the model vehicle speed for a certain period of time is calculated, and then the road-CF is determined on the basis of the mean deceleration. However, it is difficult for this method using the mean deceleration to estimate the road-CF immediately after the vehicle is braked.
According to an electrically operated vehicle, i.e., the vehicle operated by an electric motor, if the anti-skid operation is initiated when the vehicle is running on the road of a relatively low coefficient of friction, the wheel speeds of driven wheels will be oscillated at a certain period which is determined in dependence upon the type of vehicle. That is, in the case where the electrically operated vehicle is braked on the road surface of the relatively low coefficient of friction, and where the coefficient of friction between the tire and the road surface decreases after it reached its peak value, the wheel acceleration is greatly decreased, because the inertia moment of the tire in its rotating direction is smaller than the inertia moment of the motor for driving the road wheels. On the contrary, the inertia of the motor is relatively large. Therefore, a rotational speed differential (rotational angular velocity differential) is caused between the rotational speed (nm) of the motor and the rotational speed (nd) of the differential gear. In accordance with the rotational angular velocity differential, the torque is transmitted to the motor so as to decrease the rotational speed of the motor. However, since the inertia of the motor is relatively large, the rotational speed of the motor is not decreased so much, whereas the torque is provided by its reaction to rotate the tire (in a direction for reducing the braking torque). Consequently, the wheel speed, which once tended to be decreased, tends to gain the speed again, thereby to cause a vibration or oscillation of the wheel speed. When the vehicle is braked on the road of a relatively high coefficient of friction (high-.mu.), the oscillation still occurs. In this case, however, even if the braking torque is decreased by the reaction force, the tire torque is relatively large on that road surface. Therefore, the motion of the vehicle is not so much affected by the coefficient of friction of the road surface, until its position in a coefficient of friction-slip rate characteristic comes to be located in such a region as being almost flat to reach the peak region. Accordingly, it is possible to distinguish the road conditions of the high-.mu. and the low-.mu., on the basis of the oscillating state of the wheel speed of the road wheel running on the high-.mu. road and that of the road wheel running on the low-.mu. road.